This invention relates in general to the art of pickling aluminum articles and more in particular to a new process for pickling the same.
Articles of aluminum are often pickled at their surfaces in order to remove undesired oxide layers. This pickling operation may serve to obtain a more attractive appearance but also to give the pickled surface better characteristics for making adhesive joints or welded joints or to prepare it for an anodizing treatment.
The pickling operation is usually carried out by immersion of the article into an acid bath which comprises an oxidation agent in addition to a strong acid. Baths of chromic acid and sulfuric acid, as well as baths of sodium bichromate and sulfuric acid have been found to be most suitable for this purpose. However, although excellent results may be achieved in this way, the use of such pickling baths has become more and more objectionable in these days in view of the fact that the exhausted baths, due to their chromic acid or bichromate content, are most harmful to the environment and may, therefore, not be discharged to a sewer or to surface water in a direct way.
During a search for alternative pickling methods, it has been found that the same good results as in the past may be achieved with a pickling bath of sulfuric acid alone, provided that a slight anodic control voltage be applied to the bath. However, such anodic control voltage will complicate the method and will always cost energy. Therefore, it would be preferable if a method could be found wherein the control voltage is not needed.
The invention has for its object to provide a pickling process for aluminum articles wherein the use of chromic acid or bichromate in the pickling bath may be eliminated without any need for an anodic control voltage.
A further object is to provide a pickling process for aluminum articles wherein pickled articles of excellent quality can be obtained without the use of chromic acid or bichromate in the pickling bath and without any anodic control voltage.
In accordance with the present invention, these objects are realized by providing a pickling bath containing sulfuric acid without chromic acid or a bichromate oxidation agent. An aluminum article to be pickled is positioned as an anode into the pickling bath together with a carbon cathode, whereupon the anode and cathode are interconnected by an external short-circuit connection. An electric current of sufficient value will flow through the short-circuit connection and will initiate an electrochemical dissolution phenomena at the surface of the aluminum anode. As a result thereof, chemical pickling by the sulfuric acid bath is intensified in an efficient way. When the short-circuit connection is maintained for a time period of sufficient length, a complete pickling of the aluminum article may be achieved in spite of the fact that the pickling bath does not contain chromic acid or bichromate and in spite of the absence of an anodic control voltage.
According to the present invention, it is possible to obtain substantially equal results as with baths of chromic acid-sulfuric acid or bichromate-sulfuric acid. An important advantage is that chromic acid or bichromate are no longer required and this means that problems inherent to the disposal of exhausted baths are notably reduced. Another advantage is that an anodic control voltage is no longer required and this means that the apparatus may remain rather simple and that no energy supply is needed .